Stoopid Goldfish Q'

Obvious to me now, being a mum&dad to 2 goldfish that I keep for my Granddaughter is somewhat a scientific voyage of discovery.
I reckon now that I could become a water scientist..
Tonight, the task..
remove approx 25% of the water from the fishtank.. BUT methinks, its also time I changed the carbon filter and cleaned the sponge filter in the filter pump.. to do this I have to clean the sponge within the tank water that I have removed.. so I clean all the pump and change the carbon filtration.. this is airated granular carbon gravel like stuff.
Somehow, DISASTER STRIKES, a small lump of carbon has found its way into the fishtank and upon descending slowly to the bottom...STOOPID the Goldfish eats it...
Does anyone know what will happen to the Goldfish ?